The number of swine flu cases in Mexico is stabilizing. In the U.S., though more people are being diagnosed with the virus, cases have been mostly mild, claiming two lives. And health officials have backed off on closing schools where students are sick.

It may seem as though the threat of the virus known as H1N1 has lessened. But infectious disease experts and public health officials agree: The worst is likely still to come. In pandemics of the past, flu that arrived in the spring hit harder come fall, when influenza season returned.

"If you were just to bet on the odds, you would bet H1N1 would abate in the summer and return in the winter," said Dr. William Schaffner, chairman of preventive medicine at Vanderbilt University School of Medicine in Tennessee.

"The illness produced, so far, is really quite mild. But the question would be - as it circulates among humans in the Southern Hemisphere [in their winter flu season] - could it pick up a virulence gene ... that is capable of producing severe disease?

"Influenza is surprising. Its behavior is very difficult to predict."

Health officials are looking to countries such as South Africa and Argentina to see how their flu seasons progress, whether swine flu circulates there and whether the virus mutates into a nastier strain. No matter, experts are bracing for a rocky fall and winter.

An ordinary flu season kills 36,000 in the U.S. each year, hospitalizing hundreds of thousands and sickening millions. Those figures are based upon an illness that many people are immune to, either through annual vaccination or through immunity acquired by getting the flu in a previous winter.

But humans have never encountered this virus, and there is no immunity to it. Even if the swine flu stays mild, "everyone's susceptible," said Andrew Pekosz, who studies the flu at the Johns Hopkins Bloomberg School of Public Health.

Of the 36,000 figure, he said, "we can easily see that double: twice as many hospitalizations and twice as many deaths."

If the strain becomes more virulent, those statistics could be even worse.

Federal and global health officials, knowing that, are moving quickly to develop a vaccine against the swine flu. At the same time, vaccine makers are already under the gun manufacturing the seasonal flu vaccine that also will be ready by fall.

For now, no decision has been made as to whether a swine flu vaccine would be mass-produced and widely distributed. But the scientific process, which takes time, is under way even as health officials learn more each day about the H1N1 virus.

"I think we should be moving ahead as fast as possible with the vaccine," Dr. James D. Cherry, a pediatric infectious diseases expert at the University of California- Los Angeles, told a group of pediatricians at a conference Monday in Baltimore. "This is real. ... This is going to happen. We need vaccination to control this. Antivirals [medication] will not be enough to control this."

He also worries that people who come down with the flu will be more susceptible to secondary infections that could be lethal.

Researchers know that very little influenza circulates in the summer. There are many schools of thought as to why (more time spent outdoors, perhaps), but one reason seems to be that the virus transmits better from person to person in low humidity and cooler temperatures.

Officials still don't know a lot about the new virus. What they do know is that the strain is not as deadly, for now, as the Spanish influenza pandemic of 1918 that killed so many millions - though that flu started out fairly mild in the spring and returned with a vengeance during flu season.

At this point, officials with the Centers for Disease Control and Prevention have seen very little mutation. If the virus remains stable, a vaccine would likely be effective.

But Pekosz said the longer the flu strain stays in circulation, the more chance it has to change and become better at infecting people and better at making them sicker. It is unknown how the virus will interact with human proteins and every time it enters a new person, there is a chance that it will evolve to create a more serious illness.

"The prudent measure is to prevent exposure if at all possible," he said.